1. Field of the Invention
The invention relates to the field of separating metals from each other by extraction.
2. Description of the Prior Art
Several of the currently used extraction agents meant for liquid-liquid extraction are acid by nature, i.e., they are capable of releasing hydrogen ions which can be replaced by metal ions according to the following equation, in which HA indicates an extraction agent of this type, Me indicates the metal of valency n and aq. or org. indicates that the species concerned is present in the aqueous or organic phase respectively EQU Me.sub.aq.sup.n.sup.+ + nHA.sub.org .revreaction. MeA.sub.n.sbsb.o.sbsb.r.sbsb.g + nH.sub.aq.sup.+
When the extraction agent thus binds metal which is thereby transferred from the aqueous phase into the organic phase which contains the extraction agent, an equivalent number of hydrogen ions is simultaneously released into the aqueous phase and the pH of the aqueous phase decreases. It is generally known that when the acid extraction agents discussed above are used, the transfer of metal from the aqueous phase into the organic phase according to the above reaction is highly dependent on the pH of the aqueous solution and that each metal has a very limited pH range within which the extraction is effective. When the pH of the aqueous phase decreases as shown above during the extraction and the pH of the initial solution cannot be raised before the extraction section as this will cause precipitation of metal hydroxides, the effectiveness of the extraction decreases as the acidity increases, and finally a state of balance is reached, at which time the aqueous phase may still contain considerable amounts of the metal to be removed, depending on the content of this metal in the initial solution.
Thus it is easy to understand that the entire capacity of the extraction solvent cannot be utilized. So far, attempts have been made to eliminate this disadvantage in the following ways:
First, by treating only weak metal salt solutions with metal contents of 1-2 g/l at the most, in which case the change of the pH of the aqueous phase remains minute.
Second, by converting, at a separate stage, the acid extraction agent into a salt form with an alkali, in which case other ions, such as sodium, ammonium, calcium or magnesium ions, are released instead of hydrogen ions. The conversion of the extraction agent into a salt form, however, requires an additional treatment stage and often causes the formation of a third phase, and additives such as alcohols are needed to eliminate this. In addition, when a salt form of the extraction agent is used, its selectivity often decreases.
Third, by separating part of the extracted aqueous phase, by adding a neutralizing agent into it, and by returning this neutralized part of the aqueous phase into the mixing chamber after settling (U.S. Pat. No. 3,479,378), or by adding a neutralizing agent to the aqueous phase between the extraction stages. In these methods the addition of the neutralizing agent is not controlled on the basis of the reactions which take place in the mixing space but by, for example, measuring the pH of the aqueous phase which results from the settling of the dispersion. Because the number of hydrogen ions produced during the extraction depends on the amount of metal transferred into the organic phase, these indirect methods are not satisfactory, expecially if the metal content of the solution coming to the extraction stage varies.
Fourth, by producing new extraction agents which are effective at a lower pH (Cf. General Mills, Inc. series LIX 63, LIX 64, LIX 70), but then the re-extraction of metals from the organic phase becomes more difficult.
U.S. Pat. No. 3,507,645 relates to a separation process wherein a neutralizing agent is added before or during extraction, but there is no discussion of pH control during extraction on the basis of measurements made with electrodes present in the dispersion of a mixing chamber. U.S. Pat. No. 3,532,490 to Burkin mentions the use of salt forms of acid extraction agents or adding a neutralizing agent to a dispersion. But Burkin does not teach pH regulation based on direct determination of electrode potentials in the dispersion.
Hazen U.S. Pat. No. 3,676,106 discusses promotion of the extraction reactions of acid cationic exchange agents by extracting acid produced into an amine phase from which the acid is recovered. Thus, acid removal rather than neutralization is contemplated by Hazen. U.S. Pat. No. 3,251,646 describes prior known methods of maintaining pH of an extraction process within a desired range but does not mention the use of pH electrodes in a mixing chamber during extraction. It should be noted that it in laboratory tests usually is easy to reach a desired pH value using a calculated addition of the neutralizing agent required.
Swanson U.S. Pat. No. 3,428,449 relates to a copper extraction process wherein the pH of an aqueous solution is limited to the range 1.7 - 3.0 and only raising the pH if it approaches the range 1.4 - 1.5. But Swanson contemplates recording pH values determined for his aqueous phases of the settlers in a three-stage extraction process. There is no suggestion of regulation based on measurements made by pH electrodes permanently placed in the dispersion in a mixing chamber.
U.S. Pat. No. 2,782,151 relates to a method of testing oils by taking a small test sample and mixing it with sufficient water to produce a water-continuous dispersion comprising an external water phase with small droplets of oil dispersed therein. This reference requires the use of a dispersing agent to promote an oil-in-water emulsion in order to operate and furthermore, it is said to be essential that the dispersing agent imparts such characteristics to the aqueous phase that said aqueous phase tends to wet the surface of the glass electrode whereby oil coating of the electrode can be prevented. Coating or poisoning of glass electrodes is recognized as a danger to be avoided.
For some reason the prior art references discussed above have not suggested the possibility of direct measurement and control of pH by means of conventional electrodes, such as glass/calomel electrodes, permanently immersed in the liquid in mixers of an extraction process. The references show avoidance of direct measurement and roundabout procedures.